on the Mineral Phases in

Transcription

1 ISIJ Internatinal, Vl. 33 (1 993), N, 4, pp ffect f RawMaterial Cmpsitin Lime-fluxed lrn re Sinter n the Mineral Phases in Li-Heng HSIHand J. A. WHITMAN1) R&DDepartment, China Steel Crpratin, Kahsiung, 81233, Taiwan. The University f Sheffield, Sheffield. S1 4DU, U.K. (Received n ctber. 1992; accepted in final frm n January 22. 1) Department f ngineering Materials, /993) In this study small tablet specimens f pwderedcmmercial irn res mixed with fluxes and kalin were heated in a reducing atmsphere and then cled slwly in air t simulate industrial sintering. The effect f different re and flux cmpsitins n the phases fund in the sinter bnd was An studied. increase in basicity (CaISi,) favured the frmatin f calcium ferrite and densificatin f the bnd at a lw sintering temperature. The amunt f calcium ferrite decreased as the Al., cntent decreased and the amunt f rexidized hematite prduced n cling decreased. A decrease in Al., cntent prmted a densificatin f the bnd. An increase in Mgcntent, intrduced by additin f dlmite, decreased the amunt f calcium ferrite slightly, WhenMgwas added in the frm f serpentine the calcium ferrite cntent increased cnsiderably. Increase in Mg markedly reduced the amunt f rexidized hematite but prduced a less dense bnd. Generally the samecmpsitin and sintering temperature prduced similar mineral cmpsitins in the bnd independent f the irn re used. The nly exceptin waswhenan re with carse dense hematite was used, in which case the bnd cntained sme unreacted hematite. The prsity f the bnd did vary with the type f re used. Limnitic re prduced a bnd with clsed pres and less rexidized hematite at lwer temperature and basicity than did hematite res. KYWRDS: agglmeratin; sintering; irn re; simulatin; mineral phases, l. Intrductin Theprperties f irn re sinter are cntrlled by the cmpsitin and distributin f the mineral phases present. Therefre the effects f raw materials upn the minerals frmed in sinter have been extensively investigated by experiments in which raw material tablets were heated in air t simulate industrial sintering.14) In industrial sintering the raw materials are heated in a reducing atmsphere and then cled in an xidizing atmsphere. The authrs5,6) have fund that the gas atmsphere used in sintering has a significant effect n the mineral phases that are frmed. During heating f the sinter mix under a reducing atmsphere calcium ferrite dissciates int magnetite, the silicate melt and xygen, whereas under a high xygen ptential it dissciates t frm hematite and the silicate melt. When cled in air the magnetite frmed during heating may react with the silicate melt and xygen t frm calcium ferrite r else xidize t hematite. The hematite des nt tend t react with the silicate melt t generate calcium ferrite. Because f the imprtance f the gas atmsphere t the frmatin f the mineral phases in sinter the previus wrk which used nly air may nt relate well t industrial sintering. The authrs have shwn in a previus study5) that heating a tablet f the apprpriate raw materials in a reducing atmsphereand then cling in air prvides a reasnable simulatin f industrial sintering. In industrial sintering the fine particles adhered n carse res are reacted t frm the sinter bnd. In this investigatin the same simulatin experimental methd is used t determine the effect f re and flux f different cmpsitins upn the mineral phases prduced in the bnd during sintering. The results are cmparedwith thse prduced by cnventinal sintering. 2. xperimental Prcedure 2.1. RawMaterial and Tablet Preparatin Table I shws the chemical cmpsitin and size f the raw materials used in this study. All the res with the exceptin f re were hematite, re was limnitic. ach re was screened r crushed t btain a particle size smaller than 0.mmbefre use. The raw material mixtures which gave varius chemical cmpsitins f sinter were madeint tablet specimens ft sintering experiments. The rati f each raw material in the mixture was calculated by use f a simple mass balance prgram. Cylindrical tablets 6mm in diameter and apprximately 6mm in height were prduced by pressing 0.4g C 1993 ISIJ 462

2 ISIJ Internatinal, Vl, 33 (1993), N, 4 Table l. The infrmatin f raw materials. Raw materials re A re B re C re D re re F Limestne Silica sand Serpentine Dlmite Kalin Chemical cmpsitin ("/.) Size distributin ("/*) Minerals TFe Al3 Ca Mg , Si02 Ig lss l l , l0.8. l 41,l l3. -,.09 mm mm 0.09 mm ,l , , l 6. l 56, l ( mm 00/0) mm ( 00/.) mm 0"/) (Very fine pwder) Main Hem Hem Hem Hem Ge, Hem Hem Minr r Gangue Ge, Qz, Ka Ge, Qz, Ka Ka Qz, Ka Sil, Qz Qz Hem: Hematite Ge : Gethite Qz : Quartz Ka : Kalinite Sil : Sillimanite Vacuum pump Therrncup[e Thermcuple fr cntriling ternperature / water/ c[ing Gas ut[et Furnace / A[umina reactin Cmbustin bat / Water cling -- Sillcne it bubbler Air N, ( C[ing gas) tube Sampiing bar \ / Drying unit N, C C, (Reactln Air gas) Fig. 1. Sintering apparatus. f raw material mixture bnded with 8 / Water int a cylindrical muld fr I min. A pressure f 0.45 kg/mm2 was applied by a vertical laded pistn. Befre the tablet was used fr sintering it was dried at Il'C fr 3 h Sintering The sintering apparatus was the sameas that used in the authrs' previus study5,6) and is shwnin Fig. 1. It cnsists f a tube furnace in which a gas mixture was passed thrugh the alumina reactin tube (.4mm diameter by 700 mm lng) t cntrl the sintering kept at a predetermined atmsphere. The furnace was temperature fr any experiment. The specimen was placed in a small fireclay cmbustin bat sme 72 mm ng by 16mmwide by mm deep. This was pushed int the ht zne f the furnace; the specimen was at 290 frm the end f reactin tube. After the mmaway bat had been in the furnace fr the desired time it was withdrawn and culd be cled in tw different ways. Rapid cling was achieved by withdrawing the bat int an unheated regin f the tube surrunded by a water cling cil. The cling rate f specimen itself was apprximately 7'C/min in the first minute. Slw 463 cling entailed an intermediate hld at 11'C, 170 r 180 frm the end f reactin tube with mmaway the maximumsintering temperature 10 r 1 2'C, fr 2min befre remval t the cl regin in the tube. The temperature in the tube was cntrlled by a Pt/Pt-13'Rh thermcuple held at mmfrm the cmbustin bat. The temperature prfile in the tube and sample fr a given cntrl temperature had been measured. The sintering temperature was the mean f that measured at the specimen tp surface and that l mm belw the tp surface f the specimen. Typical heating and cling curves fr tw sintering temperatures are shwnin Fig. 2. During the experiment the atmsphere was cntrlled by passing a gas mixture int the reactin tube. The gases used were C, C02, N2 and air. Gas frm cylinders f the individual gases was passed thrugh a drying tube (0 mm lng and mm in diameter) filled with CaS04as a dessicant. The ttal gas flw rate was typically 0ml/min and the flw rate f the individual gases was measuredby use f individual flw meters. T btain a particular cntrlled atmsphere in the furnace, the tube was initially evacuated and the gas mixture was passed thrugh the tube fr at least 8min C 1993 ISIJ

3 ISIJ Internatinal. Vl. 33 (1 993), N. 4 befre the specimen waspushed int the reactin zne. The cling gas, 0ml/min, was either air r N2 and this was led directly thrugh a gas pipc, 7.2rnm diameter by 900 mm lng, int the reactin tube sme sec befre the end f the heating perid t cmpensate fr the time taken fr gas t reach the specimen. The lead time is n the basis f the calculatin fr the change in gas cmpsitin at the place where the specimen is lcated. The exhaust gas was passed ut f the reactin tube thrugh a bubbler cntaining silicne il. Previus wrks) had shwn that a gas mixture f C0=I "/*. C02=2/0, and N2=7/0 during heating fllwed by slw cling in air clsely simulated industrial sintering. This mixture was used as the standard fr mst f the experiments reprted here. A few tests were perfrmed with ther gas mixtures and the mixtures used will be defined when the experimental results are presented. Rapid cling enables the minerals present at the end f heating t be preserved and the changes induced during cling can then be studied. It had been als applied in the previus wrk.5) 2.3. Micrstructural Analysis After sintering, the specimens were muntedin epxy resin and vacuumimpregnated. Generally the specimens were plished t prvide a planar sectin Imm belw and parallel t the riginal tp surface f the pellet. These sectins were plished by using silicn carbide paper t I grit using ethanl as a lubricant. A final plish t 0.pm was btained using diamnd paste. Care was taken t keep the specimen free frm water thrughut the whle prcess. The specimens were examined using a light micrscpe in reflectin. The vlume prprtins f phases in sinter were estimated by using the pint cunting methd. Arund I pints were cunted fr the whle plished surface f a specimen. The vlume prprtins f each phase were calculated as fllws: :: cl (D c H Heating stag,e / / (HL / / / Rapid cling Cling stage / Slw cling Vlumeprprtin f c( phase ("/) Prsity Pints fc( phase x 0 (/,) Ttal pints f all phases ("/) = Ttal pints f all 3. Results Pints fpres = phases + Pints f pres 3.1. ffect f Basicity n Mineral Phases x 0 (/) Tablets madefrm mixtures cntaining re Cwere heated in the standard gas mixture and then cled slwly in air t simulate the frmatin f sinter bnd in industrial sintering. Results fr the prprtins f mineral phases are shwn in Fig. 3 as a functin f basicity. Thebasicity (Ca/Si02) ranges frm I.3 t 2, l fr a Cacntent in the sinter bnd f l0,/,. A narrwer range f basicity I.7 t 2.1 was cvered at a Cacntent f 13.5"/. An apprximately linear * '2 : ' e,g.s, ;h 1",,e C' ale c9 s. :9,, 8C A I Ca*I 0.5 / ; Ca / ; Ca0=13.5 / ; 1 0'c,4' 10'c,4' 12'c,3' Fig Max. sintering temperature 10'C Max, sintering temperature 12'C I 12 Time (min) Typical temperature/time prfiles f sintering experiments. Fig 'S; * , Basicity (CaISi02) 3',4': Heating time 3min, 4min ffect f basicity (CaISi02) n prprtins f phases in the bnd. The bnds have the same levels f Al3 (3,0"/), and Mg (0,1 "/). C 1993 ISiJ 464

4 ::::ae?::: ISIJ Internatinal, Vl. increase in the amuntf calcium ferrite was prduced as the basicity increased the minimumamunt f calcium ferrite being just greater than l"/* and the maximumbeing between and 60 */*. The increase in calcium ferrite was accmpaniedby a decrease in the amuntf glassy silicate frm abve t belw "/*. Interestingly the amunt f silicate was greater at a given basicity in the specimens with higher Ca cntent; the decrease in silicate with increase in basicity was again apprximately linear. The ther changes were in the rexidized hematite which decreased frm abut t less than l/, as the basicity increased. This variatin was nt linear and the decrease was steeper at lw basicities. Prsity als decreased as the basicity increased but this was nly fund ver a limited range at lw basicity; at high basicities the 'D e I B2 = 2, B2 = 1,e 33 (1993), N. 4 prsity was apprximately cnstant. All these bservatin were madeat a cnstant level f 3.00/0 A13and 0./. Mgand at temperatures f either 12 r 1 0'C. The influence f basicity at different temperatures n the phase prprtins is quite cmplex and is shwnin Fig. 4. This shwsthe effect f tw levels f basicity 1.6 and 2,1 ver the temperature range 10 t 1 0'C upn the phase prprtins. The levels f Si02, A13 and Mghave been kept cnstant. The prprtin f calcium ferrite tends t drp as the temperature is increased with higher levels f calcium ferrite being assciated with the higher basicity. Magnetite increases as the temperature increases with the greater difference fund at the lwer basicity. Fr rexidized hematite at high basicity there is a steady decrease with increase in temperature but at lwer basicity the maximumrexidized hematite is fund at an intermediate temperature. At bth levels f basicity the prsity decreases with increase in temperature, The higher basicity can reduce the prsity t a lw level at a lwer temperature.,1). 3 :c q) H 'C, A 12'C, e 12'C, l 10'C, 3' 4' 3' 4',1) (D CCD C.',D Ie :. ; q, l N, q) a: ::,e : -,,, 12 :h c",b ca 5 8 q, S Q : 1, x a: e : *5; * i 0'c 12'c 12'c 10'c 3min 4min 8min 4min Heating cnditin B2: Ca/Si02 Fig. 4. ffect f heating cnditin and basicity (Ca/Si02) n prprtins f phases in the bnd. The bnds cntain the same levels f Si02 (6.5"/), Al3 (3.0 l), and (0.1 /), Mg GL 1, Ab03 cntent ('/.) Fig. 5. ffect f Al3 cntent n prprtins f phases in the bnd. The bnds have the same levels f Ca (13.1*), Si02 (6./0), and Mg (0.1 /.). C 1993 ISIJ 465

5 ISIJ Internatinal, Vl. 33 (1 993), N ffect f Alumina Cntent n Mineral Phases Three levels f alumina were investigated ranging frm I t /0. A range f temperatures was als used. The phase prprtins btained as a functin f alumina cntent are shwn in Fig. 5. The amunt f calcium ferrite increases as the alumina increases, but at a smewhatdifferent rate in different specimens. Glassy silicate decreases as the alumina increases but the prprtinal change is muchsmaller than fr the Table 2. ffect fal3cntentand sintering temperature n mineral cmpsitins f the bnds during the heating stage. Phases Unreacted hematite ('/,) Magnetite ('/,) Calcium ferrite Glassy silicate ("/,) Relict flux ('/,) Prsity ("/,) ('/,) l 180'C Al3 3' l = Al Specimens 12'C l 180'C l 2'C l = * The specimens were heated fr 4min t I 180 r 12'C and then cled rapidly in nitrgen. The bnds have the same levels f basicity (Ca/Si02=2.1) and Mg (. I "l). calcium ferrite. Less silicate is fund at lw temperatures. Rexidized hematite als generally decreases with an increase in alumina, again the verall change is less than fr the calcium ferrite althugh at lw temperature there is an increase in rexidized hematite with increase in alumina. There nly appears t be an effect f alumina n prsity in the specimens held at the lwest temperature and this gives a rapid rise in prsity as the alumina is increased. The influence f the alumina cntent in the verall sintering cycle is quite cmplicated but the effects f alumina n the phases prduced at lw temperatures during heating can be seen frm specimensthat were rapidly quenched in nitrgen. This infrmatin is in Table 2frm which it is seen that at very lw temperatures there is a large amunt f calcium ferrite frmed in a very prus matrix at I 180'C. Heating further t 12'C dramatically reduces the amunt f calcium ferrite which is replaced by magnetite and glassy silicate with an assciated reductin in the prsity. The lwer level f alumina leads t higher magnetite and silicate levels and a very muchlwer level f prsity at the higher temperature. There is a further effect f alumina n the frmatin f rexidized hematite during the cling f sintering which may be seen in Fig. 6. Figure 6(A) shwsa sample that has been heated t 'C in an atmsphere xygen partial pressure f I x l02 atmspheres?:ce Q) e I A, B2=1.6;1 0'C,4';Serp. B2 =2.1 ;1 21 'C,4':Serp. B2 =2. i :i 2'C,3':Serp. B2 =2.1 ;1 0'C,4';Serp. B2=2.1 :1 0'C,4';Dl. :QQ 12 (D 5 : J: :1)_ ), C Fig.,B (A) Al3=3 / (B) Al3=1.00/0 H: Rexidized hematite M: Magnetite F: Calcium ferrite /u L-J 6. ffect f Al3 cntent n the frmatin f rexidized hematite during the cling stage f sintering. The specimens were heated fr 4min t 'C in the gas mixture 02=1"/, N2=99"/. (P,= 1x l,2 atm) and then cled slwly in the samegas mixture as heating. The bnds have the same levels f Ca (13.5 "l), Si02 (6./.), and (.1 "/.). Mg Fig, 7. laa :h,n Mgcntent ffect f Mgcntent n prprtins f phases in the bnd. The is Mgcntent increased by the additin f serpentine r dlmite. All bnds cntain the same levels f Si02 (6./0), and Al3 (3'/.). ('/.) C 1993 ISIJ 466

6 ISIJ Internatinal, Vl. (1 / 02 and 990/0 N2) and then slwly cled in the same atmsphere. There is n rexidized hematite in this sample which has an alumina cntent f /0. A similar sample but with alumina at / a I level subjected t the same thermal cycle cntains a significant amunt f rexidized hematite in the micrstructure, as shwn in Fig. 6(B). It seemsthen that increase in alumina reduces the frmatin f rexidized hematite during the cnventinal cling f sinter ffect f Magnesia n Mineral Phases Serpentine and dlmite were bth used as surces f magnesia in this wrk. The effect f increasing magnesia n the prprtin f mineral phases in sinter 33 (1993), N. 4 is shwn in Fig. 7. The prprtin f calcium ferrite increases with increase in magnesia under mst experimental cnditins althugh the effect is less at high temperatures and high basicities. The increase In magnesia remarkedly reduces the prprtin f rexidized hematite, which may under smecnditins be ttally suppressed. The prsity under sme cnditins increases with increase in magnesiacntent. Althugh the influence f magnesia des nt appear t be quite as cmplex as that f alumina it has a rle bth during the heating f the sinter and during the subsequent cling. In the initial stages f sinter frmatin at 1180'C increase in magnesia retards the reactin that prduces calcium ferrite, Fig. 8. After heating t 1 2'C fllwed by rapid quenching the increase in the amunt f magnesia frm 0.1 t 2.00 by additin f serpentine dramatically reduces the amunt f calcium ferrite, as is shwnin Fig. 9. The dminant phase at high magnesiacntents is magnetite. During cling, even slw cling in an atmsphere f xygen, the magnetite is nt xidized t hematite when the magnesia cntent is high. The magnetite may hwever be cnverted int calcium ferrite n cling under xidizing cnditins and the kinetics f this reactin seem t depend n whether the magnesia is added as dlmite r as serpentine. Fig. 8. (A) Mg0=0,l/,Al3=/0 (B) Mg0=2.00/0, Al3=/0 (C) Mg0=0./0,Al3=/0 H: Unreacted hematite M: Magnetite F: Calcium ferrite 5 /v!jj ffect f Mgcntent n micrstructure f the bnd at a very lw sintering temperature during the heating stage. The specimens were heated fr 4min t I 180'C and then cled rapidly in nitrgen. The bnds have the same levels f Ca (13.5 /), and Si02 (6.5 l). (A) Mg0=0.1"/ (B) Mg0=2.00/0 M: Magnetite C: Calcium ferrite S: Glassy silicate Fig. 9. ffect f Mgcntent r the frmatin f minerals during the heating stage. The specimens were heated fr 3min t 12*C in the standard gas mixture (C0=1"/, C02=24"/, and N2=7/.) and then cled rapidly in nitrgen. The bnds have the same levels f Ca (13.5"/), Si02 (6./.), and Al3 (3.0 "/) ISIJ

7 ISIJ Internatinal, Vl, 33 (1993), N ffect f res n Mineral Phases Tablets were madefrm six different res t give sinter specimens with a range f basicities frm I.3 t 2.1 but with fixed cntent f alumina (3.00/.) and magnesia (0.1"/.). These were then sintered in the standard atmsphere at several different temperatures. The results f these experiments are shwn in Fig. l. Cmparing the tests perfrmed at 12'C (SM2 & SM3) it is seen that the majr factr influencing the phase prprtins is the basicity. At a basicity f 2,l the predminant phase is calcium ferrite, there is nly a limited amuntf rexidized hematite and a significant amuntf magnetite. In these experiments the effect f re type is very slight, but the prsity f the bnd varies with the type f re used in a range frm 6 t 21 /. The lwer basicity f 1.6 prduces a very much greater amunt f rexidized hematite and a reduced amuntf calcium ferrite and almst n magnetite and a cnsiderably higher prsity (arund /*), again the effect f re type is slight. At this samebasicity f l.6 hlding at a higher temperature 10'C (SMl) gives muchthe sameamuntf calcium ferrite, but much less rexidized hematite xidized frm the magnetite and muchlwer prsity. Again, the prsity varies with the type f re used. In this case the re type is significant in that the specimen madefrm re Fwhich was Itself Is a carse dense hernatite cntains mre unreacted hematite. The influence f re type is mre nticeable in the specimens f basicity.6 I held at l 2'C (SM4) the different res prduce a difference in the prprtins f magnetite and rexidized hematite. The mst nticeable effect being fr re which has a small amuntf rexidized hematite xidized frm the magnetite. This re is different in type than the thers in that it is limnitic. The same effect is seen at a basicity f I.3 held at a temperature f 10'C (SM5) in these specimensthere is very little calcium ferrite and significantly mre silicate than in any f the ther specimens. Again re prduces muchless rexidized hematite than d the ther res ffect f Al3 and Mgn Mineral Phases al]d Prperties f Industrial Sinters The wrk dne n small tablet specimens has been expandedt a cmmercial scale by the use f sinter pt tests. Irn res, Iimestne, serpentine, dlmite, silica sand, A13pwderand cke were mixed and sintered in a 0 mm x 0 mm sinter pt t study the effects f Mgand Al3 n the phase distributin and the sinter prperties. Fr the alumina wrk the sinters all had the same prprtins f Ca (9.). Si02 (5./0) and Mg (1.9 /). Three different res were used and in ne (re C) the magnesia was added as serpentine but in all the thers dlmite was used. The results are t be fund in Fig. Il. There is a clear trend fr the calcium ferrite t increase with alumina increases as fund in the reacted tablets the abslute amunts f calcium ferrite are smewhatlwer in these industrial sinters. The hematite bth ttal and rexidized decreases as the alumina increases. Again the trend f rexidized hematite is similar t the tablet specimens. The serpentine cntaining specimens shw the same trends but cntain higher amunts f calcium ferrite. The sinters madefrm re B cntain lwer amuntsf calcium ferrite and ttal hematite but mre rexidized hematite. Theprsity fund in the pt tests is generally higher than that in the tablets and fr res Cand D sintered with a lw cke rate there is a slight increase in prsity as the alumina increases. Fr re B sintered with a high cke rate the prsity is greater and the prsity is reduced slightly as the alumina cntent increases. The pt tests prvided sufficient sinter t perfrm the standard sinter prperty tests. The tumbler index test shwed a clear trend t reduce the index as the alumina increased; the serpentine cntaining sinters (re C) shwedthe higher index. The trend in the RDI (reductin degradatin index) test was nt s clear and the re type made a great difference. The re madewith serpentine, re C, shwed a remarkable increase in alumina and re D shwed a less RDI with increase in SM1 SM2 SM3 SM4 SM5 B2 =21 12C 4 B2 =1.6; 12'C, 4' B2 :1.6 B2 =1.3 12'c, 3' 10'c, 4' SMlt SM5: xperimental serial number B2: CaISi02 3',4': Heating time 3min, 4min 1 P.' (D Ca J:: CL H M F ABCDFABCDFABCDF lrn res lrn res lrn res G A C A C lrn res lrn res P: Prsity M: Magnetite F: Calcium ferrite G: Glassy silicate H: Rexidized hematite, except re F bnd cntaining sme unreacted hematite Fig.. ffect f irn re n prprtins f phases in the bnd. The bnds have the same levels f Al3 (3'"/) and Mg (, I l). C 1993 ISIJ 468

8 :.*_*. ISIJ Internatinal, Vl. 33 (1993). N. 4 : 5 : 'D S: C H?5e q),1) :S. 'p CC :,! :, i :h *5; Fig. 22 Il re B: Di. re C; Serp. I re D; D[ \.\ e A h ::' a, cc =.,Q a, x, e e l */' re B; re C: re D: Dl. SenD Dl Ahq, cntent (') AI cntent (y.) 23 ffect f Al3 cntent n phases and prperties f the cmmercial sinter. All sinters have the same levels f Ca (9./0), Si02 (5.), and Mg (1,90). The re C sinters were added by serpentine as the surce f Mg cnstituent. The thers were added by dlmite. marked tendency t increase RDI with increase in alumina. The re B had an apprximately cnstant RDI at tw levels f alumina. There was a slight tendency fr the cke rate t increase with increase in alumina but differences in re type were significant with re B again shwing a difference in that a much higher cke rate was required. The re made with serpentine, re C, required the lwest cke rate. T investigate the effect f Mgn pt sinters, re C was used and the Mgwas added in the frm f serpentine and this cntained an alumina cntent f 0.9 /. Anre mixture was als used and in this case the magnesia was added in the frm f dlmite and the alumina cntent was 2./.. Fr bth mixes the Ca (9.5 /) and the Si02 (5.5 /) were kept cnstant. The effect f variatin f Mgn the phase prprtins is shwn in Fig. 12. The amunt f ttal and rexidized hematite bth decrease as the Mgcntent is increased. The abslute levels fr the tw res are quite different, but this almst certainly reflects the difference in alumina cntent in the tw mixes. The trend f rexidized hematite is the sameas that fund in the tablet experiments but the rexidized hematite levels in the pt sinters are higher. There is very much less calcium ferrite in the pt sinters but the trends are the sameas in the tablet experiments in that in the re with serpentine the amuntf calcium ferrite increases, whilst in that with dlmite it decreases slightly with increase in magnesia. The prsity was unaffected by variatin in magnesiawith the lw alumina cntaining re Chaving the lwer prsity. The tumbler index was slightly reduced fr bth res with increase in magnesia..the dlmite cntaining re shwed a mre marked tendency and the serpentine cntaining re with a lwer alumina had a significantly greater tumbler index. TheRDIvariatin with magnesia depends n the magnesia surce. With the serpentine cntaining re there was an apprximately cnstant value f RDI at bth levels f magnesia that were tested. The dlmite cntaining re with a higher alumina gave higher values f RDI at all magnesia levels and the RDI decreased as the magnesia was increased. Thecke rate increased slightly with increase in magnesiafr bth res, but in this case the serpentine cntaining re with a lwer alumina always required a wer cke rate. Again the dlmite cntaining re shwed a mre markedtendency. 469 C 1993 ISIJ

9 ISIJ Internatinal, Vl, 33 (1993), N. 4 a CD :,: C , l Mixed re; Dl. re C; Serp :aa S q) 1:, :e. x q) :: i 8 : A H e Mixed re; Dl I re C; Serp , :Ca CD V,:) a: *55 58 h 0, q) e Fig , Mgcntent (%) Mgcntent (%) ffect f Mgcntent n phase and prperties f the cmmercial sinter. The sinters madefrm mixed res (re A "/, re B 0/*, and re C 0/.) have the same levels f Ca (9,/.), Si02 (5.5 l*), and A13 (2./.); the Mgcntent is increased by adding dlmite. The sinters madefrm re C have the same levels f Ca (9./0). Si02 (5./.), and Al3 (0.9"/); the Mgcntent is increased by adding serpentlne. 4. Discussin Previus experimental wrk carried ut in air shwed that the amunt f calcium ferrite increased with basicity and AI3 cntent.1 4) The same trends were fund in this wrk. It was als recgnized that Mg culd retard the xidatin f magnetite t hematite in sinter7) a finding which is als cnfirmed in this wrk. Thepresent wrk has hweverfcussed n the different rles f additins n heating under a reducing atmsphere and n cling under an xidizing atmsphere. Whenthese individual steps are cnsidered the effect f alumina and magnesiaadditins cntrast markedly Initial Reactins n Heating f Sinter The initial reactin n heating a sinter mix under a reducing atmsphere appears t be the frmatin f a calcium ferrite by reactin betweenlime and hematite.6) It has been prpsed that the presence f alumina bth initiates the frmatin f the calcium ferrite and stabilizes it.4) Investigatin f the ternary phase diagram Al3-Ca-Fe3by Lister and Glasser8) shwedthat at a cnstant Cacntent (e,g, 13,/0) an increase in Al3 increased the dissciatin temperature f ternary calcium ferrite thus making the phase stable. mre In the quaternary phase diagram Ca-Si02-Al3-Fe3 Inue al.9) et shwed that the incngruent melting temperature f quaternary calcium ferrite increased with increase in alumina cntent at a cnstant Caand Si02 cntent. This is imprtant because the next reactin n heating f a sinter mix under reducing cnditins is the reactin f calcium ferrite t prduce magnetite and the silicate melt, this ccurs at a higher temperature than that at which the calcium ferrite frms fr any given lw xygen ptential6) and is als the mst imprtant reactin in reducing the prsity f the sinter. The balance f these tw reactins is clearly a delicate ne. At lw partial pressures f xygen the tendency t prduce the mre reduced structure f magnetite and the silicate melt is very great and may be mre imprtant than the tendency f alumina t stabilize calcium ferrite. There is a clear trend t increase the amunt f calcium ferrite in simulated sinter specimens 1993 ISIJ 470

10 ISIJ Internatinal, Vl. 33 (1993), N. 4 increase in alumina. Hwever, at the 2 "/, alumina level there is mre calcium ferrite and slightly less prsity present at lw temperature e,g. I 180'C, Table 2 and Fig. 8(C), than in a specimen with 3 "/* alumina when the mixture has been heated in nitrgen. The same effect is seen at the end f the sintering cycle in Fig. 5 where the level f calcium ferrite in the specimens heated t 1 0'C is always higher than in thse heated t 12'C. At the lw temperature f 1180'C in nitrgen the 2 "/. Ievel f alumina prduces the mst extensive reactin t calcium ferrite and by cntrast the amunt f reactin prduced by the increase in magnesiat 2 "/, is minimal. The initial reactin when the magnesia level is high is prbably influenced by the pssible reactin between Mgand hematite. This can lead t the frmatin f magnesi-ferrite starting at very lw temperatures in the regin f I 'C;)this culd retard the frmatin f calcium ferrite. This will mean that the bnd in the sinter with a high magnesia cntent will tend t be mreprus as it is the reactin t frm calcium ferrlte and the dissciatin f the ferrite t magnetite and the silicate melt that fills the spaces between the initial re particles. If a magnesia rich mixture is heated up t a higher ternperature f 12'C in the standard gas mixture and then cled rapidly, very much mre silicate and magnetite frmed than wuld be the case with the mre usual lw magnesia level. It is clear therefre that in such material a large prprtin f the calcium ferrite fund in the final sinter must frm during cling in an xidizing envirnment Reactins during Cling f Sinter At a high temperature under a reducing atmsphere the sinter mix will react t cnvert muchf the calcium ferrite t magnetite and the silicate melt in the heating stage. n cling such a mix in air, as is nrmal in sintering, a variety f reactins are pssible. Magnetite and the silicate melt may cmbine with smef the xygen t give calcium ferrite. Alternatively the magnetite may itself be xidized t prduce hematite. Again different cnstituents in the sinter affect these reactins differently. Alumina has an influence n the frmatin f rexidized hematite n cling in that at a lw alumina level f I "/* a cnsiderable prprtin f rexidized hematite is fund n cling under the standard gas mixture used fr the heating experiments. At a level f 3 "/, alumina very much less rexidized hematite is frmed and the majr phase is calcium ferrite. This verall trend is seen in Figs. 5and II but in the latter figure it is seen that the reductin in the amunt f ttal hematite with increase in alumina is mainly due td the reductin in rexidized hematite. Whenthe level f magnesia is altered the amuntf rexidized hematite is reduced as the magnesia level is increased. In the tablet firing experiments the arnunt f rexidized hematite may be reduced t zer by increase in magnesia. In the pt sinters mre rexidized hematite is prduced and this prbably refiects a different balance f reactins during heating f the sinter Factrs Influencing the Stability f Individual Phases The experiments discussed abve indicate that the regimes f reactin; phase frmatin and reactin during heatlng under a reducing atmsphere and the further phase reactins prduced during cling in an xidizing atmsphere bth cntribute t the final micrstructure f the sinter. The individual cmpunds alumina and magnesia influence the tw regimes in different ways. Cmparisn f the simulated sintering f the res with a basicity f l.6 indicates that in the final sinter after heating t 1 2'C fr all the res used the majr phases present are rexidized hematite typically f the rder f "/., calcium ferrite typically f the rder f "/* and abut "/, f glassy silicate. There is als a very cnsiderable amuntf prsity f the rder t /*, and the pres are pen in type. Frmthe bservatin f micrstructure, the rexidized hematite is present nt nly near the surface but als inside the specimen in assciate with pen pres. This suggests that in this case the frmatin f calcium ferrite frm the reactin n heating between hematite and flux and the dissciatin f calcium ferrite int magnetite and the silicate melt is nt sufficient t fill the spaces between initial re particles. This prus structure cntains a large amunt f magnetite (prduced frm calcium ferrite and reduced frm hematite re particles directly), sme undissciated calcium ferrite and the silicate melt. During cling, air penetrates thrugh the pen pres prviding a high xygen ptential in this structure. Magnetite is almst entirely xidized t hematite, except sme f it reacts with the silicate melt t frm calcium ferrite. Thus a large amunt f rexidized hematite is prduced. Whenthe samere mixtures are heated t the higher temperature l 0'C, mre silicate melt is prduced frm the dissciatin f calcium ferrite; the prsity is cnsiderably reduced and the pres are clsed. During the cling in air, the idatin f magnetite is limited t near the surface f specimens. Very much less rexidized hematite, very much mre magnetite and slightly mre glassy silicate are prduced in the bnds. The change in basicity frm I.6 t 2.1 after heating t l 2'C markedly increases the amunt f calcium ferrite t abut 60"/*. The amunt f rexidized hematite is limited t abut "/, as is the glassy silicate and there is abut "/, f magnetite. With this higher much reduced and the basicity the prsity is very pres are clsed despite the lw heating temperature and is f the rder f '/.. Reductin f the basicity t I.3 and heating at l 0'C almst ttally remves the calcium ferrite and the prprtins f magnetite and rexidized hematite then depend critically n the type f riginal re with the limnitic re giving a much greater rati f magnetite t rexidized hematite. This effect is same fund at the basicity f I.6 after heating at 12'C in which case the limnitic re cntains cnsiderably less ISIJ

11 ISIJ Internatinal, Vl. 33 (1993). N. 4 hematite xidized frm magnetite. It is bserved that althugh the limnitic re has a similar prprtin f prsity t that f ther res reacted under similar cnditins, the pres in this case tend t be clsed. Such clsed pres may well prevent the rexidatin f magnetite n cling in an xidizing atmsphere. The limnitic re appears t be mre easily fused than a hematite re and prduces a clsed pre structure lwer sintering temperatures Relatinship between Sinter Structure and Prperties The cnventinal wisdm is that the RDI increases with the amuntf rexidized hematite in the sinter as it is cntrlled by the transfrmatin f hematite t magnetite during reductin at a temperature arund 5'C.ll) In the sinter pt tests the RDI des seem t increase with reductin in the amunt f rexidized hematite if the experiments in which the alumina was varied are cnsidered. The same is nt true in the experiments in which magnesia is changed at cnstant alumina. The RDI des hwever seemt relate well t the changein rexidized hematite. As a high prsity in sinter mayprmte the transfrmatin f hematite t magnetite during reductin, the RDIwuld seemt be related t the changein prsity in the samere sinters that had a variatin in alumina. Anther factr that pssibly influences the RDI is the ease with which cracks grw in individual minerals in the sinter bnd. There was smeevidence that as the alumina cntent f certain sinters increases the RDI increases.12 14) A Vickers hardness tester was used t check this bservatin in the present wrk in re C sinters. Using a I kg lad the impressin f the hardness indenter in the sinter bnd increased in size with increase in Al3 cntent. Tumbler strength tests als shwed that the mechanical strength f sinter decreased with increase in Al3 cntent. This lwer strength cmbinedwith the increase in prsity is a factr assciated with the increase in alumina. These tw factrs cunteract the reductin in hematite that accmpaniesthe increase in alumina. The utcme is that there des nt appear t be simple relatinships between the sinter mineralgy and the mechanical prperties. In rder t establish such relatinships very much mre experimental wrk is necessary in which all the cntributry factrs are prperly measured. The results btained frm tablet sintering experiments shw that the prsity f the sinter bnd increases with increase in bth alumina and magnesia at a lw sintering temperature. The bnd madefrm re B is mre prus than re Cand re D at a high sintering temperature. The similar trends are fund in the sinters madefrm pt tests. It is als clear that the cke rate increases with increase in bth alumina and magnesia. The sinter madefrm re B requires a higher cke rate than re C and re D. Therefre, the densificatin characteristics f raw material mix fr generating the bnd seems t relate well t the cke rate in industrial sintering. at 5. Cnclusins (1) An increase in basicity (Ca/Si02) favurs the frmatin f calcium ferrite and the densificatin f the temperatures. bnd at lw sintering (2) With increase in the Al3 cntent the amunt f calcium ferrite increases and the amunt f glassy silicate decreases. The xidatin ptential frm magnetite t hematite increases t a higher level n cling and the amuntf rexidized hematite decreases. The decrease in alumina prmtes the frmatin f the bnd at lw temperatures during heating f the sintering and thus prmtes densificatin f the bnd. Alumina has its mst markedeffect during the heating f cmmercial sinter. (3) Mgincrease, intrduced by the additin f dlmite, decreases the amunt f calcium ferrite slightly. WhenMg is added in the frm f serpentine, the calcium ferrite increases cnsiderably. It des nt reduce the prsity as increased Mg the frmatin f the bnd at lw temperatures during heating. The majr effect is t reduce the amunt f rexidized hematite frmed n cling in an xidizing atmsphere. Magnesia thus has its majr influence during the cling f cmmercial sinter. des nt prmte (4) The re mixes used in this wrk generally lead t similar mineral cmpsitins f the bnd fr given sintering cnditins. The majr exceptin is when an re with carse dense hematite is used, in which case the bndcntains smeunreacted hematite. Theprsity f the bndvaries with the type f re used. Limnitic re prduces a bnd with clsed pres and less rexidized hematite at lwer temperature and basicity than d hematite res. (5) In the sinter pt trials the general influence f alumina and magnesia was fund t be similar t that in the tablet sintering. The prperties, particularly the did nt vary in a simple way with RDIf the pt sinter the variatin in phase prprtins. This is due t the increase in alumina in the bnd resulting in a decrease in the bnd strength and prbably an increase in the prsity. There are therefre tw ppsing effects that increase the RDI is ne the reductin in the hematite cntent, the ther is the increase in bnd strength and the decrease in prsity with reductin in alumina in the bnd. Acknwledgements The authrs wuld like t express their appreciatin t China Steel Crpratin, Taiwan, and Department f ngineering Materials, The University f Sheffield, U.K., fr supprting this wrk. l) 2) 3) 4) RFRNCS Y. Ishikawa, Y. Shimmura,M. Sasaki. Y. Hida and H. Tqda: lrnmaking Prc., Vl. 42, AIM, Atlanta, (1983), 17. K. It, Y. Hida, M. Sasaki and Y. Shimmura: Trans. Irn Steel Insl. Jpn., 23 (1983), B3. M. Taniguchi, K. Yamaguchi, K. It and Y. Hida: T,'ans. I,'n Stee/ Inst. Jpn,, 24 (1984), B70. P. R. Dawsn. J. stwald and K. M. Hayes: Trans. Inst. Min. Metall., 94 (1985), C71. C 1993 ISIJ 472

12 ISIJ Internatinal, Vl. 33 (1993). N. 4 5) L. H. Hsieh and J. A, Whiteman: ISIJI,It., 29 (1989), 24. 6) L. H. Hsieh and J. A. Whiteman: ISIJ Int., 29 (1989), 6. 7) G. S. Vasil'ev, G. G. fimenk, D. A. Kvalev,. I. Sulimenk and L. B. Gamazva:Stee/ USSR,I (1971), 4. 8) D. H. Lister and F. P. Glasser: Trans. Bril. Ce,'am. Sc., 66 (1967), ) K. Inue, H. Hayashi, K. Yshika and T. Takahashi: R D KbeSteel ngineering Reprts, 34 (1984), N. 4, 22. l) B. Phillips, S. Smiyaand A. Muan: J. Am. Ceram. Sc., 169 l l) l 2) l 3) 14) (1961), 44. K. Kanisawa, M. Wajima, H. Sma.. Mikuni and Y. Ith: Tetsu-t-Hagane, 67 (198 l), S48. Y. Yamaka Ș. Nagaka, Y. Yanadaand R. And: Trans. Irn Stee! Inst. Jpn., 14 (1974), 185. F. Matsun, S. Nishikida and H. Ikesaki: Trans lrn Stee! Inst. Jpn., 24 (1984),. F. Matsun, S. Nishikida and H. Ikesaki: T,'ans. Irn Stee! Insl. Jpn., (1985), ISIJ